Surface Emitting Laser (SEL) is a semiconductor structure of which laser is emitted perpendicularly to a top surface, for example, the Taiwan patent No. I268031 entitled “Vertical Cavity Surface Emitting Laser and Method for Fabricating the Same” and the Taiwan patent No. I403050 entitled “Vertical Cavity Surface Emitting Laser (VCSEL), VCSEL Array Device, Optical Scanning Apparatus, and Image Forming Apparatus”.
Referring to FIG. 1, a traditional surface emitting laser structure includes a P-type electrode 1, a substrate 2, a bonding layer 3, a stress release layer 4, an impressed current distribution layer 5, a lower reflecting layer 6, a passivation layer 7, a luminescent diode layer 8, an upper reflecting layer 9, a band-pass layer 10, and an N-type electrode 11. The lower reflecting layer 6 and the upper reflecting layer 9 are made of high-reflectance materials (optimal value: a reflectance of 100%) to form a resonant cavity. The band-pass layer 10 is capable of reflecting incident light with a wavelength in a particular range, allows the incident light with the wavelength in the particular range to pass therethrough, and includes a reflectance of 90-99% and a penetration of 1-10%. Since the wavelength of most electroluminescence generated by the luminescent diode 8 is not in the particular range, most of the electroluminescence is reflected repeatedly between the lower reflecting layer 6 and the upper reflecting layer 9 and keeps emitting on the luminescent diode 8. Then, the electroluminescence resonates with the luminescent diode 8 to generate new electroluminescence. However, only a small portion of electroluminescence generated by the luminescent diode 8 has a wavelength in the particular range and is able to emit through the band-pass layer 10.
In such a traditional surface emitting laser structure, the passivation layer 7 is only used non-conducting oxidized metals to avoid a short circuit, whereby the light extraction efficiency of the laser reduced since the electroluminescence generated by the luminescent diode 8 would leak from the passivation layer 7. Thus, it is hard to meet the user's requirement on the reflectance. Accordingly, it needs to set a larger size of the luminescent diode 8 for the poor light extraction efficiency; however it further causes the response time increased. Besides, the impressed current distribution layer 5 also needs to be a large area in order to control a flow direction of the current, thereby the effective area of the lower reflecting layer 6 is hard to increase and fails to form a favorable resonant cavity accordingly. In addition, such a traditional structure design does not generate a real vertical light source, and signals are prone to significant attenuation due to the long-distance transmission, thereby failing to satisfy the user's needs.